1. Field of the Invention
The present invention generally relates to diagnostic and test equipment for analyzing high-speed data bit streams and, more particularly, to devices and methods capable of measuring, among other things, the signal quality of a data communications receiver by delaying the effective decision point of the device under test (DUT) to perform analysis such as bit error rate, eye diagramming, mask testing and other common measures of signal quality.
2. Description of the Related Art
In recent years, the performance of a high-speed communication facility or device has risen to a point that the ability of accurately measuring its quality has become an ever-increasing challenge. In the area of data communication devices, for example, efforts to reduce size and power while increasing the throughput of a device have increased the possibility of error. Network suppliers, integrators and users want assurances that such devices will perform reliably and can accommodate data transmission rates that routinely exceed several gigabits/second. Providing such assurance requires determining the effective error rate and signal quality of the data received by a high-speed communications device.
In order to test a receiver, one needs the receiver in the testing path. Receivers of modern, high-performance channels incorporate sophisticated input signal conditioning (both intended and non-intended) as well as advanced decision circuits. Decision-making in the face of the applied signal conditioning is what must be tested. Conventional methods for testing receivers include creating a stressed input to the receiver whereby the stress is meant to emulate the “worst-case” applied input signal. Logically, if the receiver was able to operate error-free in face of this “worst-case” input signal, the receiver was assumed good. Typically, using this worst case scenario to see whether the receiver is operating error-free is a far simpler task than doing a full diagnostic analysis using sophisticated receivers built inside commercial test equipment (used to test transmitters and communications channels).
To test if a receiver is operating error-free, the result of the received decision can be sent to a bit error rate checking device and measured. A bit error rate checking device is small enough that many receiver DUTs have them built-in as they are comprised of compact exclusive-OR and counting logic, internal to the DUT. Alternatively, many receiver DUTs are actually transceiver devices (they have transmitters along side their receivers) and they support testing modes that re-transmit the received signal. This re-transmitted signal can then be connected to an outside bit error rate testing instrument for testing.
There are several commercially available testing systems that characterize and validate the performance of a data signal from a device or communications subsystem using bit error rate and eye diagramming oscilloscope measurement methods. In these systems, the analysis is done at the input to the commercial test system. This is ideal for testing transmitters and/or channels where an output of the device under test is available to inspect. However, as noted above these techniques and structures are not useful for testing receivers where no output is readily available.
Drawbacks of these conventional systems include the real possibility that the stressed-eye condition does not represent the actual worst case application signal. Clearly, then the entire assumption that a DUT was sound would be or at least could be false. Additional drawbacks of these conventional system include that they provide insufficient data to have a diagnostic understanding as to what the receiver is actually. Clearly, if one had such data many more issues would be understood and resolved and done so more efficiently. Another drawback associated with conventional systems is the inability of using known extrapolation techniques for grading performance deeper than the measurement time. For example if one could do so, it might well indicate an ability to use well-understood analysis techniques already developed for non-receiver testing for doing deeper diagnosis on receiver DUTs.
Digital receiver circuits are different than digital channel circuits. Digital receiver circuits accept an input signal from a digital channel. This input signal is an analog voltage that must be interpreted to determine what digital value was being sent. This interpretation involves looking at the voltage levels and timing present on the analog signal. One way of doing this is to use a voltage comparator to logically slice the analog input as being either above or below a pre-selected logic threshold and a D-flip flop to then sample the result of the comparator at time instants that correspond to proper bit periods of the data being carried by the analog voltage on the digital channel. Once the decision is made, the output from the decision circuit is simply a logical output (high or low) and looses all other characteristics of the analog input signal—characteristics that, if measured, provide numerous benefits in understanding and extrapolating performance. Therefore, the normal result of the digital receiver circuit hides the ever-important information needed for diagnostic evaluation by transforming the analog input signal into digital ones and zeros.
Other elements of a digital communications system (including digital channels circuits) have easy access to the analog voltages used to communicate digital information. This easy access allows for easy measurement. For example, a signal from the transmitter half of an Ethernet port can easily be measured on an Ethernet cable. In this case, the Ethernet port is the transmitter and the cable is the digital channel circuit. The voltages on the cable comprise the analog signal that is carrying the information and this can be presented to a measuring device (such as an oscilloscope) for analysis. No such convenience exists if one wants to examine the receiver half of the Ethernet Port. Analysis of the digital receiver is not possible as it is not accessible.
Typical techniques for analyzing channel circuit are not available in analyzing receiver circuits. For example techniques such as bit error rate testing, eye diagramming, jitter measurement, Q-factor measurement, eye-diagram mask testing, fast “four-corners” margin testing, frequency response, step response all depend upon the ability to manipulate the decision point of a digital decision circuit while collecting bit error or probability of occurrence information. This cannot be done on receiver circuits. It will be appreciated that specialized machines are available on specialized commercial test instruments with specialized receiver circuits intended for use only on transmitter and channel testing (not receiver testing).